Battery separator material

ABSTRACT

A battery separator material which is folded to form an envelope is heat sealed along at least two opposing edges thereof to form a battery separator. The separator material is comprised of discrete polyolefin pulp fibers in a sufficient amount to heat seal the separator along its edges. The fibers are bound by an organic polymeric binder. A process of forming the battery separator is also disclosed.

This application is a division of application Ser. No. 164,588, filedJune 30, 1980, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to battery separator material and moreparticularly to battery separators which fully envelope the electrolyticplates.

2. Description of the Prior Art

Electrolytic cells (i.e. batteries) formed by spaced apart metal platesconnected in series for storage of electric energy are useful for avariety of purposes. The plates of the electrolytic cells tend to forman active substance on the surface of the electrodes. When the activesubstance drops off, battery capacity drops, and also the degradation ofthe metal electrodes forms products including metals salts and otherconductive substances which tend to bridge between two electrodes,thereby short circuiting the cell. These salts are degraded metalproducts are primarily due to the highly acidic environment in storagebatteries.

In order to protect the electrode plates, separators have been usedbetween the plates which are porous, thereby allowing ionic exchangethrough the separator while providing adequate separation between thecells to prevent short circuiting. Typically, these separators areconstructed of cellulose fiber or fused polyolefin sheets. Exemplary ofbattery separators are those disclosed in U.S. Pat. Nos. 2,973,398;3,890,184; 4,024,323; 4,055,711; 4,113,927; 3,753,784; 3,694,265;3,773,590; 3,351,495; 3,340,100; 3,055,966; 3,205,098 and 2,978,529.

Battery separators having improved properties have been required withthe introduction of what is known as the "maintenance free battery." Themaintenance free battery is one which is a sealed unit and does notrequire the intermittent addition of water thereto. Typically, thebattery separator in a maintenance free battery is preferably anenvelope sealed on at least three sides in order to prevent bridgingwith metal salts between electrodes. A primary requirement of thesebattery separators is that they must have sufficient porosity in orderfor the ionic exchange to occur while the pores of the separator must besufficiently small to prevent the migration of heavy metal ions and,consequently, bridging which short circuit the cell.

Further requirements of a battery separator for a lead-acid battery areresistance to acid and oxidative and reductive chemical reactionsbecause of the highly acidic environment within the battery. Also,separators should have as low an electrical resistance as possible toprovide good cold discharge performance.

In accordance with the invention, a battery separator material isprovided which has excellent filtering, electrical, chemical andphysical properties which is readily fabricated and may be folded andheat sealed along the edges in the form of an envelope to act as abattery separator, particularly in a maintenance free battery.

BRIEF DESCRIPTION OF THE INVENTION

A battery separator material which is folded to form an envelope is heatsealed along at least two opposing edges thereof to form a batteryenvelope. The separator material is comprised of discrete polyolefinpulp fibers in a sufficient amount to heat seal the separator along itsedges. The fibers are bound by an organic polymeric binder.

DETAILED DESCRIPTION OF THE INVENTION

The fibers useful in the practice of the invention are those fiberscapable of withstanding strong acid, such as is present in electrolyticcells.

A major portion of the fibrous content of the battery separator materialof the invention is polyolefin pulp fibers which are synthesized fromthe polymerization of ethylene and/or propylene or mixtures thereof,such as to produce polyethylene, polypropylene orpoly(ethylene-propylene) copolymers and have a fiber diameter of up to100 microns. Preferably, these polyolefin pulp fibers have a fiberdiameter of 0.01 to 20 microns, have a softening point below 340° F.,and are up to 0.5 inch a length. The polyolefin pulp fibers are presentin a weight range from 30 to 100 percent by weight, and preferably 70 to90 percent by weight based upon the nonbinder constituents of theseparator material.

The polyolefin fibers most useful in the practice of the invention arethose which are characterized as synthetic wood pulps. These polyolefinfibers have a surface treatment which imparts wettability and ease ofdispersion in water to the fibers. Typically, the surface tension of thepolyolefin fibers is about 70 mN/m. Typical properties of thepolyethylene and polypropylene fibers are those which have: (1) aspecific gravity of less than one and more preferably between about0.900 and 0.965 kg/dm³ ; (2) a melting point of between about 250° and340° F.; (3) a yield stress of greater than 300 daN/cm² ; (4) a tensilestrength at break of greater than 200 daN/cm² ; (5) a modulus ofelasticity in tension of 7,000 to 20,000 daN/cm² ; and (6) a dielectricconstant of 2 to 4, a dielectric strength of 2 to 5×10² kv/cm and atransverse resistivity of 10¹⁵ to 10¹⁸ Ωcm.

The polyolefin pulp fibers comprise a major portion of the batteryseparator material for several reasons. Among the reasons are that theyare substantially inert to acidic conditions such as are present inelectrolytic cells and, secondly, they have the desired softening pointof below 340° F. so that the fabricated battery separator can be heatsealed along at least two opposing edges thereof to form an envelope.They have low ohmic resistance due to their diameters. Further, thepolyolefin pulp fibers have sufficient flexibility so that the finalbattery separator material can be folded and worked while providing goodenvelope integrity and ease of processing on papermaking equipment.

In addition to the polyolefin pulp fibers, staple glass fibers may beincorporated into the battery separator material in order to impartrigidity and tensile strength while maintaining the inert chemicalcharacteristics and low ohmic resistance of the battery separator.Preferably, the glass fibers useful in the practice of the inventionhave fiber diameters less than 20 microns as the mean diameter. Thefibrous component as glass constitutes up to 60 percent by weight basedon the nonbinder content of the separator material and preferably 5 to15 percent by weight. Exemplary of the glass fibers useful in thepractice of the invention are the glass microfibers, i.e. those havingfiber diameters of 0.20 to 4.0 microns. These glass fibers may be ofpreferred compositions known as soda-lime borosilicate or C glass withexcellent chemical durability sold by Johns Mansville Corporation.

In addition to the polyolefin pulp fibers and the glass fibers,polyester and/or polyolefin staple fibers and/or cellulose pulp fibersmay be incorporated into the battery separator material. These polyesterand/or polyolefin staple fibers preferably have a denier of 0.5 to 1.5and are incorporated at a level of up to 30 percent and preferably 5 to15 percent by weight based on the nonbinder constituents of theseparator material.

In addition, fillers may be used to reduce the pore size of the batteryseparator material. The fillers are used at a level of up to 60 percentand preferably 40 to 60 percent by weight based upon the nonbindercontent of the separator material. The fillers useful in the practice ofthe invention are those which are particulate fillers having an averageparticle size of 0.02 to 20 microns, such as kaolinite, halloysite,montmorittonite, tinite and illite which are all clays, and otherfillers such as silica, quartz, calsite, luminite, gypsum, muscavite,diatomaceous earth and the like. In addition to the inorganic fillers,organic fillers having a particle size of 0.2 to 50 microns may also beused for the same purpose as the inorganic fillers. These organicfillers are typically inert thermoplastic organic polymers such ashydrocarbon polymer powders. Typical polymers are polystyrene andpolyolefin polymers and copolymers. The fillers reduce the ohmicresistance and pore size along with the cost of the battery separatormaterial.

The fibers and filler are bound together by an organic binder preferablysupplied as a latex or aqueous dispersion. Preferably, the binder is thepolymer of monoethylenically unsaturated monomers. "Monoethylenicallyunsaturated," as used herein, is characterized by the monomer having a>C═CH₂ group. These monoethylenically unsaturated monomers are, but notlimited to, the acrylic monomers such as methacrylic acid, acrylic acid,acrylonitrile, methacrylonitrile, methylacrylate, methylmethacrylate,ethylacrylate, ethylmethacrylate, acrylamide and the like; olefinhydrocarbons such as ethylene, butylene, propylene, styrene,alpha-methylstyrene and the like; and other functional unsaturatedmonomers such as vinyl pyridine, vinyl pyrollidone and the like.Typically, these polymers are acrylic polymers dispersed in water at alevel of 30 to 50 percent by weight and are in the form of a latex.Additionally, the polymer should be film forming.

Although the polymers useful in the practice of the invention may beprovided with sufficient functional groups to self-crosslink, i.e.crosslink without the addition of other materials, crosslinking agentsmay be added to provide the required crosslinking characteristics.Preferably, the polymers will crosslink at a temperature below 200° F.and in a preferred range of 150° to 190° F.

Crosslinking agents suitable in the practice of the invention includealdehydes such as formaldehyde, glyoxal, acrolien and the like;synthetic resin precondensates obtained by the reaction of an aldehydegenerally with compounds containing nitrogen, like dimethylol urea,dimethylolethylene urea, di- and trimethylol triazon dimethyluron, di-and trimethylol melamine and other cyclic or noncyclic, water soluble ornon-water soluble precondensates of urea and melamine formaldehyde. Thereactive methylol groups may be blocked or partially blocked by alcoholshaving 1 to 4 carbon atoms. Apart from the above, other knowncrosslinking agents too may be used, such as diepoxides and epichlorinderivatives thereof, dichlorophenols, beta substituted diethylesulfones, sulfonium salts, N-methylolacrylamide and methylacrylamide andderivatives thereof, diisocyanates and the like. Up to 4 percent of thecrosslinking agent may be incorporated into the binder composition. Toomuch crosslinking agent may render the sheet material too brittle orhard and not provide the required flexibility for forming the batteryseparator.

The use of an organic binder for binding the polyolefin pulp fibers toform a battery separator material is a substantial improvement over theprior art. While the prior art teaches that polyolefin fibers ofcontinuous or long staple characteristics can be heat fused to form aseparator material such heat fusion requires complicated and expensiveprocessing. To the contrary binding of the polyolefin pulp fibers withan organic binder avoids these complicated and expensive processingsteps by providing a battery separator material which can be fabricatedon standard papermaking equipment. In addition, the battery separatormaterial in accordance with the invention shows increased strength andelongation over prior art pulp fiber separator material, thus providinga material with improved integrity and improved handling and fabricationproperties.

The battery separator material which is formulated in accordance withthe invention has a preferred porosity between 50 to 70 percent, ascalculated by the following equation:

    %E=(1-ρ.sub.a /ρ.sub.f)100

where E is porosity, ρ_(a) is apparent density of the battery separatormaterial and ρ_(f) is the columbic density of the material.

Further, the battery separator material has an ohmic resistance ofbetween 0.001 ohms in² to 0.025 ohms in² and more preferably below 0.020ohms in².

The battery separator material of the present invention may be formed onstandard papermaking apparatus such as a fourdrinier, an inclinedfourdrinier, a cylinder machine, a rotoformer or the like. A typicalprocess involves charging the fibrous constituents along with the fillerand a large quantity of water to a pulper. The pulper disperses thefiber and filler in the water. Various polyelectrolytes and surfaceactive agents may be added to provide the appropriate chemical andphysical properties in accordance with standard papermaking techniques.The fiber and filler slurry is then charged to the machine chest of afourdrinier papermaking machine. From the machine chest the slurry istransferred onto the moving wire of the fourdrinier and water is allowedto drain therefrom. After the draining of the water by gravity, suctionis applied to the wet web to further remove water from the web. The webis then pressed between felt rolls. After wet pressing, the wet webpasses through a saturator, where the binder is applied. Finally, thesheet is dried on drying cans. The drying temperature required isbetween 230° to 260° F. During the drying cycle, the binder may becrosslinked if it has sufficient functionality to do so, or there is acrosslinking agent in the furnish or in the binder. It is preferred thatthe drying be conducted at a temperature below the fusion temperature ofthe polyolefin fibers so as to maintain them as discrete fibers ratherthan fused fibers. The sheet material is then calendered to a uniformthickness and may be embossed on one side thereof and wound on rolls. Inthe alternative, the binder can be applied to the dry web and redried.Preferably, the battery separator material has a thickness of 0.005 inchto 0.040 inch. These materials may be used for enveloping or may be cutto the appropriate size and used as a leaf separator per se.

Preferably, the separator material is embossed on one side thereof and arib is formed of extruded polyolefin plastic on the opposing side of theembossment. The rib is applied by extrusion of molten polyolefin resinonto the separator in a generally cross-sectionally triangularconfiguration through a die.

In forming the rib, low density polyolefin pellets are fed into thehopper of an extruder which is in communication with a single piece diehead. A series of polyolefin strands of circular cross section areextruded through the die lip attached to the front of the die head. Thestrands are aligned and directed onto the cooled female portion of thedie roll pair just prior to the nip formed between the profiled femaleand the male die. At the nip the polyethylene strand is contacted withthe separator web and the profile of the rib is established, togetherwith the emboss on the back web of the separator material. Contact ismaintained with the cooled female die roll to insure solidification andrelease of the profiled rib. The polyolefin strand is at a sufficienttemperature to soften the pulp fibers in the web, thereby fusing the riband the web into a unitary construction.

The triangular configuration of the rib provides necessary spacingbetween the electrolytic plate and the separator material to allow forcirculation of the electrolytic solution about the plate. Further, thetop of the rib aligns with the embossment on the opposing side so thatthere is a guide for true rolling of the battery separator materialduring winding subsequent to production and unwinding for fabricationinto battery separators. Preferably, the height of the triangular crosssection is about 0.010 to 0.100 inch with a base correspondinglyconfigured to generally form an equilateral triangle.

In producing the battery separator from the battery separator materialwhich is wound on reels, the reel is unwound and fed into an envelopingmachine which folds the battery separator material around theelectrolyte plate and heat seals it along opposing edges thereof. Theelectrolytic plate may also be inserted into a preformed envelope andthe enveloped electrolytic plate is mounted within a battery casing.

The invention is illustrated by reference to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of the battery separator of the invention;

FIG. 2 is an end cross-sectional view of the battery separator of FIG.1;

FIG. 3 is a cross section taken through one of the ribs shown in FIGS. 1and 2;

FIG. 4 is a front view partially in section of the battery separatormaterial of FIG. 1 on a roll; and

FIG. 5 is an exploded view of a portion of the roll of FIG. 4.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIGS. 1 through 5, wherein like reference numeralsrefer to like parts, there is shown a battery separator materialgenerally designated by the numeral 1, which is composed of a continuousweb formed as previously described with a plurality of polyolefin ribs 3of generally triangular cross-sectional configuration on one side 7 ofthe web 2. On the opposing side 4 of the web 2 there is provided aplurality of embossed grooves 5 which are adapted to receive the ribs 3when the battery separator material is wound on a roll 6, as is shown inFIG. 4. The embossment 5 is configured so as to receive the apex 8 ofthe rib 3 so that when the material is rolled the apex 8 of the rib 3and the embossment 5 mate to form an interlocking arrangement for truerolling. More specifically, FIG. 5 shows the interengagement of theembossment 5 and the rib 3 when the material is wound on a roll.

The invention will be further illustrated by the following example.

EXAMPLE 1

A furnish composed of the following ingredients was prepared:

    ______________________________________                                        Ingredient        Parts by Weight                                             ______________________________________                                        Pulpex EA         80                                                          (by Hercules                                                                  synthetic wood pulp -                                                         avg. fiber diameter                                                           4.9 microns)                                                                  Microglass 112    15                                                          (glass microfibers                                                            sold by Johns Mansville)                                                      Dacron 1/4 inch,   5                                                          1.5 denier                                                                    ______________________________________                                    

The above furnish was charged to a pulper and 24 parts water was addedthereto. The material was agitated in the pulper until a uniform slurrywas prepared. The slurry was pumped to the machine chest of afourdrinier papermaking machine.

The slurry was pumped to the head box of the fourdrinier. The slurry wasfed onto the moving screen of the fourdrinier, water was allowed todrain by gravity, and then suction was applied. The wet web wassaturated in a saturator with 11 parts by weight of a self-crosslinkingfilm forming acrylic latex sold under the trademark Polymerics 1440having a pH 5.45 and a weight average molecular weight of about 10⁶ and0.11 parts be weight of Aerosol OT (sodium dioctyl sulfosuccinate) anionic surfactant by American Cyanamide. The wet web was then dried bypassing it over heated rolls to impart a temperature of 260° F. to theweb.

The sheet material from the fourdrinier was calendered and embossed toform linear indentations longitudinally oriented on the sheet material.A triangularly cross-sectioned polyolefin rib of low density polyolefinhaving a melt index of 5 g/10 min. at 300° F. (manufactured by RexenePolyolefins Co.) having a base of 0.036 inch and a height of 0.034 inchwas extruded on the material opposing the indentations.

The material was wound as a roll with the apex of the triangular ribnested in opposing indentations on the material.

The material was unwound and fed into a Dale enveloping machine whichcut the material to a predetermined size, folded each piece in half andheat sealed opposing open edges of the envelope.

The battery separator had the following properties:

    ______________________________________                                        Property          Value                                                       ______________________________________                                        Backweb t.sub.b ins                                                                             00.013                                                      Overall t.sub.o ins                                                                             00.038                                                      Weight/area g/m.sup.2                                                                           89.000                                                      Air porosity, 1/4"                                                            orifice, secs.    93.100                                                      Ohmic Resistance                                                              20 mins Ω in.sup.2                                                                        00.019                                                      Ohmic Resistance                                                              24 hours Ω in.sup.2                                                                       00.015                                                      Apparent density.sup.1                                                                          00.377                                                      % Porosity.sup.1,2                                                                              67.200                                                      Pore diameter avg.                                                            in microns        08.700                                                      % Volume greater                                                              than 20 microns   11.000                                                      % Columbic density                                                                              01.100                                                      Tensile strength lbs.                                                                           04.600                                                      % Elongation      06.500                                                      ______________________________________                                         .sup.1 Measured on an Aminco Mercury Porosimeter under liquid mercury         pressure.                                                                     ##STR1##                                                                     -   The battery separator fabricated in accordance with Example 1 was used     in a lead-acid storage battery and tested for performance. The battery     tests were run according to the Battery Counsel Industry (BCI) recommended     specifications for vehicular, ignition lighting and starting types. The     results are as follows.

    ______________________________________                                        Discharge Performance Characteristics                                         ______________________________________                                        Reserve Capacity                                                              25 amps at 80° F.                                                      minutes to 10.5 volts 109 minutes                                             Cold Crank                                                                    450 amps at 0° F.                                                      voltage at 30 sec.    7.25 volts                                              ______________________________________                                    

EXAMPLE 2

A furnish composed of the following ingredients was prepared:

    ______________________________________                                        Ingredient      Parts by Weight                                               ______________________________________                                        Pulpex EA       50                                                            Hi Sil 233      40                                                            (silica)                                                                      Dacron 0.75 denier                                                                            10                                                            ______________________________________                                    

The above furnish in hand sheet form was processed and the binderapplied similar to the production machine, Example 1, except that theionic surfactant was eliminated. The battery separator was formed inaccordance with Example 1 and had the following properties:

    ______________________________________                                        Property           Value                                                      ______________________________________                                        Backweb t.sub.b ins                                                                              0.011                                                      Weight/area g/m.sup.2                                                                            92.8                                                       Porosity 1/4" secs.                                                                              176                                                        Ohmic Resistance   0.009                                                      20 min Ω in.sup.2                                                       Ohmic Resistance   0.008                                                      24 hours Ω in.sup.2                                                     Apparent density   0.337                                                      % Porosity.sup.4   63.4 (calculated max.                                                           73.5%)                                                   Pore diameter avg. 13.9                                                       diameter microns                                                              Tensile strength lbs.                                                                            6.1                                                        % Elongation       15.9                                                       ______________________________________                                         .sup.4 Limited by porosimeter range                                      

As is demonstrated by Example 2, a battery separator material having ahigh filler loading has excellent physical and electrical properties.

As can be seen from the foregoing description of the invention, abattery separator material is provided which can be easily fabricatedinto a battery separator having excellent chemical, physical andelectrical properties.

Although the invention has been described with reference to specificcompositions and processes, the invention is only to be limited so faras is set forth in the accompanying claims.

I claim:
 1. A roll of battery separator material having one side thereofembossed to form a plurality of longitudinally extending indentationsparallel and laterally spaced across the width of said material and onthe opposing side thereof having a plurality of ribs of polyolefinmaterial extruded thereon, each of said ribs having a substantiallytriangular cross section and positioned in alignment with an indentationand having an apex in mating engagement therewith to prevent lateralslipping of laps of the roll and provide alignment for feeding off theroll.